U.S. Pat. No. 4,220,708 (Heller) describes inter alia a series of 3-furyl and 3-thienyl fulgides and fulgimides having the general formula (I) below: ##STR1## wherein R.sup.1 represents an alkyl, aryl or aralkyl group,
A represents oxygen or &gt;N--R.sup.6, in which R.sup.6 is alkyl or aryl, PA0 B represents oxygen or sulphur, PA0 R.sup.2 and R.sup.3 independently represent an alkyl, aryl or aralkyl group or one of R.sup.2 and R.sup.3 represent hydrogen and the other an alkyl, aryl or aralkyl group. Alternatively, ##STR2## may represent an adamantylidene group, X represents hydrogen or an alkyl, aryl or aralkyl group and each Y is independently selected from hydrogen, halogen, alkyl, aryl, aralkyl, alkoxy and aryloxy group. PA0 R.sup.2 and R.sup.3 independently represent an alkyl, aryl or aralkyl group or one of R.sup.2 and R.sup.3 represents hydrogen and the other an alkyl, aryl or aralkyl group or the group ##STR4## represents an adamantylidene group, X represents hydrogen or an alkyl, aryl or aralkyl group, PA0 Y represents an alkyl or aralkyl group and PA0 R.sup.4 represents an alkyl, aralkyl or aryl group. PA0 R.sup.1 represents an alkyl group, preferably a lower alkyl group having 1 to 5 carbon atoms, e.g. methyl, ethyl, propyl or butyl; PA0 R.sup.2 and R.sup.3 are the same or different alkyl groups especially lower alkyl groups having 1 to 5 carbon atoms, including cycloalkyl groups such as cyclopropyl, PA0 R.sup.4 is a lower alkyl having 1 to 5 carbon atoms, e.g. methyl, ethyl, propyl or butyl, phenyl or substituted phenyl, e.g. p-tolyl, and X and Y are a lower alkyl groups, e.g. one of those enumerated above, phenyl or a substituted phenyl group, e.g. an alkyl substituted phenyl group).
The compounds described in the above U.S. patent exhibit improved photochromic properties compared with previously known photochromic compounds.
In particular they show an increased sensitivity and response due in part to the coloured species absorbing less strongly and acting less as an internal filter at wavelengths used to promote photocolouration.
The colouring process takes place by subjecting the compound to near ultraviolet radiation (eg 366 nm) and photobleaching takes place in the visible spectrum below 600 nm. The optimum conditions are with green light of 514-550 nm. The advantageous properties of these 3-furyl and 3-thienyl fulgides may be summarised as follows:
1. These compounds show slow photochemical reversal in some cases, allowing near non-destructive read-out at wavelengths below 600 nm.
2. They have good thermal stability.
3. They exhibit high conversions into the coloured forms.
4. Photochemical decay products do not interfere with the photochromic properties.
These photochromic compounds are therefore suitable for a wide variety of uses, including optical information storage and display systems but have the limitation that the coloured forms absorb only very weakly radiation above 600 nm so that they are not suitable for optical read/write/erase systems which use helium-neon lasers or laser diodes, which emit light at wavelengths longer than 600 nm.